This invention relates to an anti-lock control system for vehicle wheel brakes in which the actuator for establishing the braking pressure is provided by operation of an electric motor.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reducing vehicle stopping distance and deterioration in the lateral stability of the vehicle.
U.S. application Ser. No. 789,576 filed on Oct. 21, 1985, now U.S. Pat. No. 4,664,453, and assigned to the assignee of this invention describes a wheel lock control system for preventing the wheels of a vehicle from locking up while being braked. In this system, the wheel brake pressure that results in the wheel slip being at the critical slip value and which produces substantially the maximum braking force between the tire and the road surface is identified. When an incipient wheel lockup condition is detected, the brake pressure so identified is then applied to the wheel brake so as to substantially continuously establish the critical slip value between the wheel and the road surface resulting in the maximum possible braking effort.
The brake pressure producing substantially the critical slip value and therefore substantially the maximum braking force is identified in the above system by repeatedly calculating the braking force between the wheel and the road surface during braking based on an equation defining the motion of a free body consisting of the wheel, tire and the brake. This equation utilizes measured values and system constants that are based on, for example, brake lining coefficient of friction and area and wheel radius. The measured brake pressure corresponding in time to the peak calculated force is stored. When an incipient wheel lockup is detected indicating that the critical wheel slip value establishing the peak braking force between the wheel and road surface has been exceeded, the stored brake pressure is the pressure that produced substantially the peak braking force. After detection of an incipient wheel lockup condition, the process of identifying the brake pressure producing the peak braking force is ended and the brake pressure is dumped to allow the wheel to recover from the incipient wheel lockup condition. When recovery is sensed, the stored brake pressure that produced substantially the peak braking force is reestablished to establish a braking condition in which the wheel slip is substantially at the critical slip value for the existing road-tire interface condition.
The wheel brake pressure is controlled in the aforementioned system by means of a brake pressure control actuator that includes a reversible electric motor and a motor driven actuating mechanism. The actuating mechanism is operatively connected to a piston in a cylinder which is reciprocally movable in the cylinder to increase and decrease the displacement volume in the cylinder and therefore generate brake actuating pressures therein when the actuator is actuated. Also, in the aforementioned system, the control of the electric motor to establish braking pressures is established based on the measurement of the braking pressure established by the actuator and applied to the brakes of the wheel.